The present invention relates to a process for preparing aromatic nitro compounds by reacting aromatic compounds with nitrating acids comprising HNO.sub.3 and, if appropriate, H.sub.2 SO.sub.4 and/or H.sub.3 PO.sub.4 and/or H.sub.2 O, the said process being carried out in the presence of surface-active substances.
Nitro compounds of the most varied types are important intermediates for preparing plastics, dyes, auxiliaries, pharmaceuticals and other chemicals.
There is abundant technical and scientific literature, including patent literature, on the preparation of nitro aromatic compounds by various processes involving isothermic or adiabatic conditions, batchwise or continuous operation and various reactors. Products required in small amounts are preferably prepared in batchwise operation, whereas mass products, such as nitrobenzene, nitrotoluene and nitrochlorobenzene, are preferably prepared by continuous operation. Suitable reactors for batchwise operation are, in general, stirred tanks, whereas, for example, tubular reactors are preferred for continuous operation. In the case of products produced in great quantities, there has been no lack of attempts to recover the considerable heat of reaction at a high temperature level and to utilize it for other purposes, for example for concentrating the waste acid. More recent and promising adiabatic operations have been described, inter alia, in EP-A 668 263 and EP-A 675 104. Whereas the processes of the EP patent applications just mentioned already have a high level of successful resource utilization (high material yields and high energy recovery), it is still important, especially for mass products, to attempt to increase resource utilization even more.
In the context of the preparation of 1-nitroanthraquinone, JP 06/293709 mentions the use of di-(2-ethylhexyl)-sulphosuccinic acid Na salt. This process is characterized by a purely organic reaction medium (1,2-dichlorethane) and the use of NO.sub.2 or N.sub.2 O.sub.4 in combination with SO.sub.3 as nitrating agent. The di-(2-ethylhexyl)-sulphosuccinic acid Na salt is employed in an amount of 0.17 g, based on the nitrating agent, consisting of 4.1 g of SO.sub.3 and 4 g of NO.sub.2. The 1- and 2-nitroanthraquinone yield obtained does not exceed that of other examples without the use of the di-(2-ethylhexyl)-sulphosuccinic acid Na salt mentioned. In Ind. Eng. Chem. Res. 34 (1995), 4305, it is noted in the context of an investigation of the role of surface reactions in heterogenic nitrations of aromatic compounds that addition of amphiphilic impurities to the organic phase slows down the reaction; this effect was confirmed using cetyl-trimethylammonium bromide as deactivator (p. 4305, left-hand column, below FIG. 15). In this publication, the reaction conditions involve a mixed acid of 41.41% by weight of H.sub.2 SO.sub.4, 1% by weight of HNO.sub.3 and the remainder to 100% by weight of water. When the reaction is realized on an industrial scale, this phenomenon of reaction slow-down leads to a drastic reduction in the space-time yield.
Surprisingly, it has now been found that, in contrast to the observations in Ind. Eng. Chem. Res., a considerable increase in reaction rate and yield is obtained under the conditions according to the invention described further below when surface-active substances are employed. These surprising results have the following advantages: for mixing the reaction mixture, the expenses required for apparatus are lower, thus reducing the investment costs of a nitration process. The cheap surface-active substances which are prepared as detergents in mass production can be used, in particular. The surface-active substances are employed in the ppm range. It is possible to select surface-active substances having a wide range of properties, and it is likewise possible to select a wide range of other reaction conditions.